Abstract:
Twenty, primiparous, 87.5% crossbred Holstein cows were used to evaluate the effect of supplemental recombinant bovine somatotropin (rbST) and mister-fan cooling on milk production and hormonal responses relating to renal function in regulation of body fluids at early, mid and late lactation. Animals in each experiment were divided into two groups. Cows in the first group were housing in the barn under normal shade (NS) as non-cooled cows and cows in the second group were housing under normal shade plus misty-fan cooling system (MF) as cooled cows. Cows in both groups were received subcutaneous injections of 500 mg of rbST every 2 wks three times in each stage of lactation. The results showed that the averaged temperature humidity index (THI) in both NS and MF barns were from 80 to 84, but the rectal temperature (RT) and respiration rate (RR) of cooled cows were lower than those of non-cooled cows. The milk yield, DMI and water intake of cooled cows without rbST had tendency to higher than non-cooled cows. Cows treated with rbST were significantly higher in milk yield, mammary blood flow (MBF), body fluids, dry matter intake (DMI), water intake, rectal temperature (RT) and respiration rate (RR) than the pretreated period in each stage of lactation . During supplemental rbST in both cooled and non-cooled cows, a marked increase in plasma insulin like growth factor (IGF-I) coincided with an increase in the plasma aldosterone level, while there were no changes in plasma cortisol and vasopressin concentrations in all stages of lactation. The plasma thyroxine (T4) concentration was significantly decreased during early lactation. The study of renal function in both cooled and non-cooled cows whether supplemental rbST or not showed no significant changes in renal hemodynamics. There were decreases in the rate of urine flow, urinary electrolytes excretion and osmolar clearance in both cooled and non-cooled cows supplementation with rbST. The lithium clearance study revealed the increases in water and sodium reabsorption in renal proximal tubule. These data can conclude that rbST supplementation is main action to increase milk production which is mediated primarily through higher body fluid and secondary increased MBF to mammary gland for distribution of nutrients for milk synthesis. The stimulatory effects of rbST on body fluid expansion could be in part stimulate sodium and water reabsorption in renal proximal tubule by mediated via increases in plasma levels of aldosterone and IGF-1 which may involve a stimulation of renin–angiotensin–aldosterone (RAAS) system, but not for vasopressin.
